The present invention relates to a key sheet applied for a push button switches in various electronic appliances, such as information communication devices including portable telephones, automobile telephones, and audio devices.
Various electronic appliances, including mobile communication terminals such as portable telephones and automobile telephones, are required to have smaller weight, size, and thickness. Accordingly, the push button switches used in such devices are also required to have smaller size. In order to meet the above requirements, key sheets are known in the art having key tops comprising a translucent resin and a translucent resin film adhered to all the surfaces of the key tops, except for the rear surface. This kind of key sheets have been widely adopted recently.
FIG. 15 is a cross-sectional view of a conventional key sheet. A key sheet 31 comprises a key top 33, comprising a translucent resin, and a translucent resin film 32 adhered to an outer surfaces of the key top, except on its rear surface, so that one or more convex-shaped press button 34 is formed on its upper surface. A symbol layer 35 is formed on a rear surface 32a of the translucent resin film 32 to display a graphical symbol (such as characters, symbols, numbers, and patterns) on the press button 34. A graphical symbol is attached to the symbol layer 35 through printing or the like. A conventional key top 33 included a gate corresponding section 36a, which corresponds to a gate used for injection of resin in a substantially center position of a rear surface 33a. The key sheet 31 is typically manufactured by first curving a translucent resin film 32 to have a shape corresponded to a profile of the key top 33 and then injecting molded resin in the curved portion.
Recently there are needs for performing various designs on key sheets of this kind. For instance, there are needs for providing various designs to key sheets used for push button switches of portable telephones. Accordingly, key sheets are required in which graphical symbols displayed on the key sheets are easily recognized while the key sheets have high quality designs.
However, a gate corresponding section 36a is disposed at a position corresponded to a gate of a mold for injecting resin. Since the gate corresponding section is disposed in almost the center of the rear surface 33a of the key top 33 in conventional key sheets 31, the gate corresponding section 36a was visible when a key top 33 was viewed from the top surface of the press button 34. Accordingly, the conventional key sheets 31 described above were inferior in their designs. Specifically, the graphics of the symbol layer 35 were deformed because of the existence of the gate corresponding section 36a, in the case where the symbol layer 35 is formed on the rear surface 32a of the translucent resin film. In this way, visibility of the press button 34 of the key sheet 31 was reduced because the gate corresponding section 36a inhibited the viewers from easily viewing the symbol layer 35.
In order to solve the problems described above, attempts were made to provide the gate corresponding section 36a on a periphery of the key top 33, as shown by reference numeral 36b in FIG. 15. However, the gate corresponding section 36a was still visible from the upper surface of the press button 34 of the sheet when the gate corresponding section 36a was merely moved to an edge of the conventional key sheet structure.
On the other hand, Japanese Laid-Open Patent Publication 8-7698 discloses a key sheet 41, as shown in FIG. 16, in which a gate corresponding section 46a is disposed to project outwardly from a side face of a projection of a key top 43.
However, a thin key sheet 41 cannot be realized when the key top 43 was further projected from the lowermost surface of the translucent resin film 42 because the key top 43 was thicker by a thickness L of the projection 43a. Further, it was difficult to fill a molten thermoplastic resin in a cavity of a mold, when manufacturing the key top 43 of the above shape. A so-called short shot, an air pocket, and so forth were easily formed in this method.